melt-blown layer provides more surface loading so the filter is easier to pulse clean. Specialty filters, such as PTFE and spunbond filters, also are

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1 Using nanotechnology for more efficient dust collection Thomas Green United Air Specialists Inc. Even if your process dust doesn t contain nanoparticles, cartridge filters built with nanotechnology can improve your plant s dust collection system. Nanotechnology the control of matter smaller than 1 micron is a hot topic. Nanotechnology is used in products as diverse as atomically engineered food, anti-aging cosmetics, stain-resistant clothing, self-cleaning window glass, high-performance golf clubs, and information storage products. But for companies that process and handle dry bulk solids, one of the most exciting and useful applications of nanotechnology is the use of nanofiber media in dust collector cartridge filters. When used in these filters, one microscopic layer of fibers 1,000 times smaller than the diameter of a human hair can translate into huge advantages in filter efficiency, cleanability, filter life, and energy use. A typical dust collector cartridge filter is a cylinder of pleated fabric (called media) on a cylindrical frame. The filter has two endcaps, and one is attached to the dust collector s tube sheet, a frame that holds one or several cartridge filters. Dust-laden (dirty) air enters the dust collector and passes through the cartridge filter s media, leaving most of the dust on the filter s exterior (dirty side) and embedded within the media, while clean air passes to the cartridge s interior (clean side). The clean air is exhausted to the atmosphere or back to the process. The dust from the dirty air builds up on the media surface and within the media and periodically must be cleaned off to ensure that the filter can remove dust from the air. Typically, this is done with automatically timed compressed-air pulses flowing opposite to the dirty air. This cleaning method is called reversepulse or pulse-jet cleaning. Pulse-jet cleaning can t remove all of the dust particles, especially those embedded within the media. As dust builds up on and within the media, the air has to work harder to pass through the filter. The pressure of the dirty air trying to get through the media becomes higher, while the air on the filter s clean side remains low. As this pressure differential (called pressure drop) increases, more compressed air must be used to clean the filter. Eventually, cleaning becomes more and more difficult, until the filter has to be replaced. Filters galore The most common types of media for cartridge filters today are cellulose, cellulose blended with a synthetic fiber (referred to as blended cellulose), and cellulose with a nanofiber layer (also called a nanofiber filter). These filters are popular because they re all relatively inexpensive (with nanofiber filters being the most expensive) and can be used in a wide range of applications. Cellulose and blended cellulose filters are true depth-loading filters: They consist of one homogeneous media layer that captures most of the dust particles within the media, while some particles cling to the media surface and begin to form a dust cake. The filter initially captures only larger particles, but as the dust cake builds up, it reduces the media s pore size and captures smaller particles as well, resulting in more efficient filtration and cleaner filtered air. When the dust cake becomes too thick, the pressure drop becomes too high and the filter must be cleaned. The timed air pulses remove the dust cake from the surface, but can t completely clean all the dust that s embedded within the media. After cleaning, filtration resumes, with more dust captured within the media and a new dust cake forming. This process continues until the media contains so many particles that it can no longer be cleaned effectively. Then the filter must be replaced. In contrast, a nanofiber filter is a surface-loading filter: A surface-loading filter has a layer on top of the cellulose media (now called the substrate) that provides a thin, finely pored surface; this surface captures most of the dust particles and lets clean air pass through the media. Few particles migrate into the substrate. This makes the filter easier to clean because only the dust cake has to be removed. Filter manufacturers have recognized the benefits of surface loading and have devised ways to increase it. For example, some cellulose or blended cellulose filters currently on the market have an outer layer of melt-blown fibers. Melt-blown fibers are formed by extruding molten polymer and blowing it with hot, high-velocity air. The fibers are formed into a very fine web and applied to the filter media surface. Like a nanofiber layer, a

2 melt-blown layer provides more surface loading so the filter is easier to pulse clean. Specialty filters, such as PTFE and spunbond filters, also are designed to provide more surface loading, but these filters are used in only about 20 percent of dust collection applications, primarily those with extremely sticky or agglomerative dust. They cost up to three times more than cellu- Figure 1 Size comparison of melt-blown fibers and nanofibers a. Melt-blown fibers (600x) lose, blended cellulose, and nanofiber filters, so they re used only when an application has unique requirements that only these filters will handle. Comparing melt-blown and nanofiber surface layers Filters with melt-blown or nanofiber surface layers are easier to clean than depth-loading filters because their fine, fibrous webs help maintain surface loading. But nanofiber filters provide several advantages because of the nanofibers diameter and the nanofiber surface layer s thinness. The fibers. Melt-blown fiber diameters measure around 10 microns, as shown in Figure 1a. Nanofiber diameters can be more than 100 times thinner, ranging from 0.07 to 0.15 micron, as shown in Figure 1b. These ultrathin fibers form a permanent mesh-like surface with exceptionally small pores. The surface layer. A melt-blown fiber layer, as shown in Figure 2a, is about 100 times thicker than a nanofiber layer. Because nanofiber is so effective at capturing even submicron dust particles, only a very thin coating, about 0.1 to 0.5 micron thick, is needed, as shown in Figure 2b. In comparison, a melt-blown layer is about 50 microns thick. This added thickness translates to depth loading within the melt-blown layer, trapping dust particles that are very difficult to dislodge during pulse cleaning. b. Nanofibers (600x) Pore size distribution is another key difference between melt-blown and nanofiber layers. In a melt-blown layer, fibers of varying sizes entangle and overlay. This entanglement creates various-sized pores up to 40 microns in diameter throughout the layer. In contrast, the nanofiber layer has much smaller and uniformly sized pores on the filter surface. This minimizes particle penetration into the substrate. Both surface layers filter out submicron particles from the dirty air, but the nanofiber layer s tiny, uniform pores do it more efficiently. The nanofiber layer s smaller fibers, reduced thickness, and tiny, uniform pore sizes add up to a more efficient, easier-to-clean filter. The surfaceloading nanofiber layer develops a dust cake that s easily pulsed off during cleaning. Because dust isn t embedded in the media substrate, as with a depth-loading filter, a nanofiber filter stabilizes at a lower pressure drop, dramatically increasing its fractional efficiency (that is, the filter s ability to

3 remove greater numbers of particles at a given size or size range) and maximizing the filter life. Let s take a closer look at how the nanofiber filter compares to other filters in several key areas: cleaning, emissions, and filtration efficiency. Figure 2 Cartridge filter surface-loading layers a a. Melt-blown layer (160x) Cleaning Pulse cleaning is costly to a plant because it uses compressed air, which is one of the most expensive utilities in a plant and for a dust collection system. That s why a filter that requires less cleaning is desirable. A depth-loading filter requires frequent pulse cleaning, as much as 17 times more often than a nanofiber filter. Dust builds within the depth-loading filter s substrate, increasing the pressure drop and requiring the cleaning system to pulse more often to return to optimum (design) pressure drop. A filter with a melt-blown layer requires much less cleaning than a standard depth-loading filter but much more than a nanofiber filter. For example, a blended cellulose filter with a melt-blown surface layer requires pulsing more than eight times as often as a filter with a nanofiber layer. The melt-blown layer will experience significantly more dust depth-loading, making it more difficult for the pulses to remove dust and regenerate the filter to an acceptable pressure drop. Because a nanofiber layer prevents particles from passing into the media, the filter stays clean much longer, requiring fewer cleaning pulses. Over time, the reduced compressed-air usage greatly reduces the dust collector s operating costs. b. Nanofiber layer (160x) In addition, fewer pulses put less stress on the filter, extending the nanofiber filter s life. In fact, tests at various filter manufacturers labs have shown that nanofiber filters last up to twice as long as standard cellulose and blended cellulose filters without melt-blown layers. This longer life means fewer filter replacements, reduced maintenance, and less downtime for filter changeouts. a Note: Dark layer is cellulose substrate; media are shown at a 23-degree angle so that the thin surface layer can be seen. Emissions When dust is pulsed off a filter, the vast majority accumulates in the dust collector s hopper. However, an unavoidable by-product of pulse-jet cleaning is that a small percentage of the dust will be released into the atmosphere. A standard cellulose filter typically emits more than 35 times more dust into the atmosphere than a nanofiber filter. A filter with a meltblown surface layer does much better. Still, it emits two to four times more dust than a nanofiber filter does. Not only does the nanofiber filter remove smaller particles from the air, it also requires fewer cleaning pulses, so has lower total emissions.

4 Filtration efficiency The ideal filter media maintains its filtering efficiency (that is, its ability to remove as many dust particles as possible) while retaining its permeability. The more permeable, or porous, a filter is, the easier it is to pull air through the filter, resulting in a lower pressure drop. The idea of efficiently removing microscopic particles while being as permeable as possible may sound like a contradiction. Historically, filter manufacturers have struggled with this balance. Some make the media pore sizes smaller to increase efficiencies by removing even the finest dust particles, but this results in a higher pressure drop, which leads to more frequent pulse cleaning. Adding a melt-blown layer is another solution. The melt-blown layer s smaller pore sizes increase filtering efficiency, but the layer itself adds additional depth to the media, restricts airflow, and increases pressure drop. In contrast, because a nanofiber filter s surface layer is so thin and finely pored, it provides the highest filtering efficiency for bulk solids of any cartridge filter available today. In effect, this nanofiber layer does all the work, so the substrate s purpose is primarily structural. The substrate can be highly permeable, re- sulting in the perfect combination of very low pressure drop with very high filtering efficiency. In addition, because using a nanofiber filter reduces the pressure drop, the dust collection system can use a smaller fan with a lower horsepower requirement, which will also reduce electrical costs. Measuring filter efficiency For many years, manufacturers of dust collector filters for bulk solids have measured filtration efficiency by particle weight. For example, a filter with the rating 95 percent of 1.0-micron particles removes 95 percent (by weight) of 1.0-micron particles in a dust sample. More recently, as filters have become more and more efficient, manufacturers have begun to use Minimum Efficiency Reporting Value (MERV) ratings. MERV ratings were established by the American Society of Heating, Refrigerating, and Air- Conditioning Engineers (ASHRAE) for filters used in indoor air-filtration systems, which remove extremely fine particles from the air. MERV ratings are based on particle count. The ratings measure a new, out-of-the-box filter s worst-case performance in a series of tests that determine the num- ber of particles a filter removes from a dust sample at various dust loadings. The higher the MERV rating (1 to 20), the better the filter is at removing particles, especially very small particles, from the air. 1 MERV ratings are certified by independent labs. Not all nanofiber filters have the same MERV rating, nor do all cellulose and blended cellulose filters or filters with a melt-blown surface layer. But for an idea of efficiency differences, consider these examples: Some nanofiber filters have a MERV 15 rating. This means the filter is at least 85 percent efficient at capturing 0.3- to 1.0-micron particles and 90 percent efficient at capturing 1.0-micron or larger particles. Some newer filters with melt-blown surface layers may have a MERV 14 rating (75 to 84.9 percent efficient at capturing 0.3- to 1.0-micron particles and 90 percent efficient at capturing 1.0-micron or larger particles). In contrast, standard cellulose and blended cellulose filters earn MERV 10 ratings, which means these filters are rated to capture only 1.0-micron and larger particles. Putting it all together While the MERV rating is the most accurate efficiency measurement available, you shouldn t select a filter on MERV rating alone. Other criteria, including cleanability, compressedair usage, filter life, pressure drop, and emissions, are important determinants of a filter s performance and life-cycle cost. For example, a filter with a melt-blown layer may achieve a high MERV rating but operate at a higher pressure drop, have shorter filter life, and require more compressed air and electrical power to operate than a nanofiber filter. Fine nanofiber web on cartridge filter (5,000x) provides exceptional surface loading. 2 Unfortunately, no industry standards exist to measure all these factors together and clearly show how they relate to each other. The best way to select a filter for your application is to consult an expert, test filters in your equipment, and ask for referrals from others in your industry. PBE

5 References 1. Covered in ASHRAE Standard 52.2, Minimum Efficiency Reporting Values (MERV), American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), 1791 Tullie Circle Northeast, Atlanta, GA 30329; , fax ( 2. Advanced Nanofiber Filtration Technology cartridge filter media, United Air Specialists, Cincinnati, Ohio. For further reading Find more information on nanotechnology and cartridge filters in articles listed in Powder and Bulk Engineering s comprehensive Article Index under Nanotechnology and Dust collection and dust control at and in the December 2006 issue. Thomas Green is vice president and director of the Corporate Filtration Research Center of CLARCOR, the parent company of United Air Specialists Inc. and Clark Filter ( filter.com). He has a BS in chemical engineering from the University of Cincinnati. He can be reached at 4440 Creek Road, Cincinnati, OH ; , ext. 8720, fax (tomgre@ uasinc.com).